Breast cancer metastasizes to the skeleton more frequently than any other tumor type causing the devastating complications of hypercalcemia, pain, fracture and nerve compression syndromes. Parathyroid hormone-related protein (PTHrP) is a tumor product that mediates hypercalcemia through its systemic effects to stimulate osteoclastic bone resorption and renal calcium reabsorption. In addition to this role in mediating hypercalcemia, PTHrP contributes significantly to the pathophysiology of breast cancer metastases to bone in several ways: 1) Because PTHrP has powerful potent bone resorbing capacity, its expression by primary breast tumors may aid in the establishment of breast cancer metastases in bone. 2) Transforming growth factor-beta (TGFbeta), present in high concentrations in the bone microenvironment, increases secretion of PTHrP by breast and other cancers. Thus, PTHrP expression in the primary breast tumor may be a marker for the development of bone metastases and TGFbeta may further enhance PTHrP expression in bone. Blocking the effects of either PTHrP or TGFbeta may be effective treatments to prevent the development of bone metastases or to retard the growth of established osteolytic lesions. The long-term objective of this study is to determine the role of PTHrP and TGFbeta in the establishment and growth of breast cancer metastases to bone. The following specific aims and methods are proposed. An established animal model of PTHrP-expressing breast cancer metastases to bone will be used in all specific aims. 1. To determine the role of PTHrP in a) the initial establishment of breast cancer metastases in bone and b) the rate of growth of established breast cancer metastases in bone. PTHrP neutralizing antibodies will be administered a) to animals prior to tumor inoculation and b) to animals with established osteolytic bone metastases. Growth of osteolytic lesions will be quantitated radiographically with a computerized image analysis system as well as histologically. 2. To determine the role of cell heterogeneity for expression of PTHrP in a primary breast cancer cell population and compare it with that in breast cancer metastases to bone. In situ hybridization, immunocytochemistry and single cell cloning will be used to determine PTHrP expression. 3. To determine the role of TGFbeta in vivo in the development and growth of breast cancer metastases to bone. TGFbeta- responsiveness will be knocked out in a breast cancer cell line by expressing a mutant TGFbeta type II receptor that acts in a dominant negative fashion. The cell line will be studied in vivo for the ability to cause osteolytic metastases. 4. To compare general inhibitors of bone resorption (bisphosphonates) alone and in combination with factor-specific therapy (against PTHrP and/or TGFbeta) in the prevention and growth of breast cancer metastasis to bone. Using animal models osteolytic metastases, bisphosphonates will be compared with treatment against PTHrP and/or TGFbeta as in specific aims 1 and 3. Combinations of bisphosphonates with factor-specific therapy will be studied for synergistic effects.